The present invention relates to centrifuges, and more particularly, to an improved method and apparatus for measuring the thickness of sludge being deposited against the sidewall of a rotating turbine can in a centrifuge, by measuring the electrical admittance between the sidewall and the base, shaft or body of the centrifuge which are usually at ground potential.
Centrifuges are widely used in many fields of science and industry. Centrifuges use centrifugal force for various applications such as separating solids from liquids and separating heavier liquids from lighter liquids, etc. One application of a centrifuge is to separate metal and other particulate matter from lubricating oils. In this application, the centrifuge acts as a device for filtering dirty oil. Dirty oil under high pressure is inserted into the rotating turbine can of a centrifuge. The turbine can is rotating at a very high speed. Due to centrifugal force, heavier particles are repelled outwards, removed from the oil, and deposited on the sidewall of the rotating can. This results in the formation of a layer of sludge or particulate matter on the sidewall of the can which keeps increasing as the cleaning process continues. From an application point of view, it is very useful to be able to monitor the thickness of the sludge layer continuously as it is increasing during operation of the centrifuge.
The prior art discloses methods and apparatus for measuring the thickness of sludge being deposited on the sidewall of a rotating centrifugal basket or can while the centrifuge is operating. For example, U.S. Pat. No. 4,229,298 to Bange discloses a method and apparatus for determining the thickness of a charge wall being formed on the sidewall of a centrifugal basket. An electrically conductive plate is disposed in the open basket inwardly of the sidewall to provide a source of voltage for establishing an electric field between the plate and the sidewall. A capacitance sensor is used to sense a change in the capacitance resulting from the formation of a charge wall on the sidewall of the rotating basket.
The addition of the conductive plate requires a centrifuge with an open rotating basket or can. The electrode cannot be inserted into a centrifuge having a closed basket for filtering liquids under high pressure. Thus, the method and apparatus disclosed in the Bange patent is limited to centrifuges having an open rotating basket.
In the present invention, no additional conductive plate, electrode, or rod is introduced. Instead, the present invention measures the admittance (capacitance or conductance) between the turbine can sidewall and the base, shaft, or body of the centrifuge. In a typical centrifuge, the turbine can is usually electrically shorted or connected to the base, shaft and body of the centrifuge. Therefore it is not apparent that one can measure the admittance between them. This is the reason the Bange patent introduces an extra conductive electrode. The present invention modifies the centrifuge by electrically isolating the turbine can from the rest of the centrifuge structure (i.e. the combined base, shaft and body). The modification to the centrifuge is relatively simple and does not affect its operation.
Frequently, it happens that the thickness of sludge on the sidewall of the basket or can is not uniform. The method disclosed by the prior art carries out a local measurement on a small portion of the sidewall. Thus, the method measures the sludge thickness in an area opposite the fixed electrode. The method used by the present invention has an integral character and provides the average value of sludge thickness deposited on the sidewall of the centrifuge. This is parameter that is more accurate and useful than the local thickness at a certain limited region of the centrifuge.
The advantages of the present invention over the prior art are that the present invention is cheaper and simpler to implement, the invention works with minimal modifications to the centrifuge, and the prior art can only be used in centrifuges having an open rotating basket or can. The prior art method and apparatus cannot be used in cases where the rotating turbine can is closed and/or includes high pressure liquids inside it. It should also be noted that it is very difficult to place an additional fixed electrode or rod inside a rotating can. The addition of an extra conductive electrode may disturb the symmetry of the centrifuge and the thickness of the sludge near the electrode may become non-uniform. In this case, the data obtained from the device will be erroneous. In the present invention, the symmetry of the centrifuge is not disturbed.